1. Field of the Invention
The present invention relates generally to optoelectronic imaging devices, and more specifically to optoelectronic imaging devices that provide two optical images having two respective polarizations.
2. Discussion of the Related Art
In certain imaging applications, it is desirable to use dual simultaneous images of light having different orthogonal polarizations of light emanating from a particular scene. One approach splits the optical image received at the imaging system into two optical branches or paths, each branch being a separate image having orthogonal polarizations. These separate images are imaged by separate image acquisition devices, such as charge-coupled device (CCD) imagers or other detector arrays. Common devices to split the optical image include the use of linear, circular or other polarization control elements that physically separate the orthogonal states such that independent optical branches can be used to acquire the polarized images.
However, such systems suffer from the lack of inherent alignment of the two optical branches when acquiring registered images, i.e., alignment and registration of the resulting images using separate detector arrays are difficult to maintain. Misalignment of one polarization branch relative to the other can result in different magnifications, orientations, rotations, and optical aberrations from one image to its orthogonal polarization counterpart. Additionally, physically separating the optical branches used in imaging the two polarizations mandates the presence of two independent image acquisition devices.
Another approach provides a dual image detector using a single specially adapted image acquisition device, such as described in U.S. Pat. No. 5,438,414, issued Aug. 1, 1995 to Rust. In this approach, the image is incident upon an integrated dual imaging detector on a single silicon chip. The chip includes a wafer of beamsplitting material (a birefringent element) with an optical mask of opaque strips that separates portions of incident light into orthogonal polarizations interleaved across a CCD. However, this approach blocks portions of the incident light in order to split the light into orthogonal polarizations without interference; thus, the resolution of the image is reduced. Accordingly, the two differently polarized images are interleaved on the CCD. Additional processing is also required to reconstruct the interleaved images into separate images having orthogonal polarizations.
Another approach, such as described in U.S. Pat. No. 5,135,183, issued Aug. 4, 1992 to Whitney, uses a birefringent element at an entrance aperture of a telescope that splits the incident light into dual images having orthogonal polarizations which are directed to different regions of a single photodetector array. However, in order to eliminate optical crosstalk between the two images, separate polarization filters (one for each orthogonal polarization) are provided in front of the respective portions of the photodetector array. However, in such systems, radiometric performance is limited by the extinction ratio performance of these polarization filters, which is often variable depending on wavelength and angle of incidence. For example, typical plate polarization filters commonly result in 100:1-200:1 extinction, and less commonly 1000:1 extinction, which tends to be angle and wavelength sensitive. Thus, any light escaping through the polarization filter which should have been eliminated or filtered becomes a significant source of image noise.